The present invention relates to a process for the preparation of olefin polymer foams and, in particular, to an extrusion foaming process for olefin polymers using a stability control additive in conjunction with a blowing agent mixture consisting essentially of dichlorodifluoromethane and at least one aliphatic hydrocarbon, or halogenated hydrocarbon, compound having an atmospheric pressure boiling point of from about 0.degree. C. to about 50.degree. C.
In addition, the present invention also relates to an expandable olefin polymer composition containing the indicated stability control additive and mixed blowing agent system admixed therein and to polyolefin foams prepared therefrom.
It is well known to prepare olefin polymer foams by heat plastifying a normally solid olefin polymer resin, admixing such heat plastified resin with a volatile blowing agent under heat and pressure to form a flowable gel and thereafter extruding the gel into a zone of lower pressure and temperature to expand and cool the gel to form the desired solid olefin foam product.
A problem frequently encountered is that of preventing an unacceptable degree of shrinkage of partially cured foam during the aging or curing period following manufacture. During the "aging" or "curing" period the blowing agent employed is gradually diffusing out of the cells in the foam product and air is gradually diffusing into the cells in place thereof. For example, until very recently only one blowing agent (i.e., 1,2-dichlorotetrafluoroethane) was known or thought to be capable of providing sufficient dimensional stability during the curing period to permit the commercially viable manufacture of low density (e.g., 1-6 pounds per cubic foot) foams of ethylenic polymer resins.
More recently, certain technology has been developed in the area of stability control agents in an attempt to permit the obtention of commercially acceptable dimensional stability with a wider range of volatile halogenated hydrocarbon blowing agents. (See, for example, U.S. Pat. No. 3,644,230 and Watanabe et al. U.S. application Ser. No. 952,865 filed Oct. 19, 1978, now U.S. Pat. No. 4,214,054, issued July 22, 1980). Unfortunately, these techniques, while generally providing low density ethylenic polymer foams having improved dimensional stability, are somewhat limited in terms of the maximum cross-section of the foam articles that can be obtained therewith from a given type of extrusion apparatus. Accordingly, it would be highly desirable to provide an improved extrusion foaming process capable of producing low density olefin polymer foam articles having both relatively large cross-sectional areas (e.g., providing an increase in the maximum achievable foam cross-sectional area which can be obtained with a given extrusion apparatus) and good dimensional stability as well as other desirable foam properties such as closed cells of small cell size and the like.